This proposal is for a continuation of a prospective, multicenter randomized clinical trial of medical/surgical therapy to prevent TIA and stroke in asymptomatic patients with stenosis of the common carotid bulb and/or internal carotid sinus -- the site of more than 90% of hemodynamically significant carotid artery atherosclerosis. The major aim of the study is to determine whether the addition of carotid endarterectomy to aspirin (325 mg) + best medical management will reduce the incidence of TIA and cerebral infarction in asymptomatic patients with hemodynamically significant carotid stenosis. Secondary objectives are to determine: (1) the surgical success in lesion removal and incidence of recurrent carotid stenosis following carotid endarterectomy, (2) the rate of progression (or regression) of carotid atherosclerosis in the medically treated group and in the control carotid arterial system, and (3) the incidence of coronary artery events (angina pectoris, non-fatal and fatal myocardial infarction) during follow-up. Quality assurance is a major component in study design. The results of the North American Symptomatic Carotid Endarterectomy Trial have demonstrated that carotid endarterectomy is the treatment of choice for symptomatic carotid artery stenosis exceeding 70%. It is estimated that the symptomatic subset represents only 10% of the prevalence of carotid stenosis. Therefore, it is all the more urgent to determine whether similar results apply to asymptomatic patients.